Patch "random: invalidate batched entropy after crng init" has been added to the 4.11-stable tree

@@ -1,6 +1,9 @@
 /*
  * random.c -- A strong random number generator
  *
+ * Copyright (C) 2017 Jason A. Donenfeld <[email protected]>. All
+ * Rights Reserved.
+ *
  * Copyright Matt Mackall <[email protected]>, 2003, 2004, 2005
  *
  * Copyright Theodore Ts'o, 1994, 1995, 1996, 1997, 1998, 1999.  All

@@ -762,6 +765,8 @@ static DECLARE_WAIT_QUEUE_HEAD(crng_init
 static struct crng_state **crng_node_pool __read_mostly;
 #endif
 
+static void invalidate_batched_entropy(void);
+
 static void crng_initialize(struct crng_state *crng)
 {
 	int		i;

@@ -799,6 +804,7 @@ static int crng_fast_load(const char *cp
 		cp++; crng_init_cnt++; len--;
 	}
 	if (crng_init_cnt >= CRNG_INIT_CNT_THRESH) {
+		invalidate_batched_entropy();
 		crng_init = 1;
 		wake_up_interruptible(&crng_init_wait);
 		pr_notice("random: fast init done\n");

@@ -836,6 +842,7 @@ static void crng_reseed(struct crng_stat
 	memzero_explicit(&buf, sizeof(buf));
 	crng->init_time = jiffies;
 	if (crng == &primary_crng && crng_init < 2) {
+		invalidate_batched_entropy();
 		crng_init = 2;
 		process_random_ready_list();
 		wake_up_interruptible(&crng_init_wait);

@@ -2019,6 +2026,7 @@ struct batched_entropy {
 	};
 	unsigned int position;
 };
+static rwlock_t batched_entropy_reset_lock = __RW_LOCK_UNLOCKED(batched_entropy_reset_lock);
 
 /*
  * Get a random word for internal kernel use only. The quality of the random

@@ -2029,6 +2037,8 @@ static DEFINE_PER_CPU(struct batched_ent
 u64 get_random_u64(void)
 {
 	u64 ret;
+	bool use_lock = crng_init < 2;
+	unsigned long flags;
 	struct batched_entropy *batch;
 
 #if BITS_PER_LONG == 64

@@ -2041,11 +2051,15 @@ u64 get_random_u64(void)
 #endif
 
 	batch = &get_cpu_var(batched_entropy_u64);
+	if (use_lock)
+		read_lock_irqsave(&batched_entropy_reset_lock, flags);
 	if (batch->position % ARRAY_SIZE(batch->entropy_u64) == 0) {
 		extract_crng((u8 *)batch->entropy_u64);
 		batch->position = 0;
 	}
 	ret = batch->entropy_u64[batch->position++];
+	if (use_lock)
+		read_unlock_irqrestore(&batched_entropy_reset_lock, flags);
 	put_cpu_var(batched_entropy_u64);
 	return ret;
 }

@@ -2055,22 +2069,45 @@ static DEFINE_PER_CPU(struct batched_ent
 u32 get_random_u32(void)
 {
 	u32 ret;
+	bool use_lock = crng_init < 2;
+	unsigned long flags;
 	struct batched_entropy *batch;
 
 	if (arch_get_random_int(&ret))
 		return ret;
 
 	batch = &get_cpu_var(batched_entropy_u32);
+	if (use_lock)
+		read_lock_irqsave(&batched_entropy_reset_lock, flags);
 	if (batch->position % ARRAY_SIZE(batch->entropy_u32) == 0) {
 		extract_crng((u8 *)batch->entropy_u32);
 		batch->position = 0;
 	}
 	ret = batch->entropy_u32[batch->position++];
+	if (use_lock)
+		read_unlock_irqrestore(&batched_entropy_reset_lock, flags);
 	put_cpu_var(batched_entropy_u32);
 	return ret;
 }
 EXPORT_SYMBOL(get_random_u32);
 
+/* It's important to invalidate all potential batched entropy that might
+ * be stored before the crng is initialized, which we can do lazily by
+ * simply resetting the counter to zero so that it's re-extracted on the
+ * next usage. */
+static void invalidate_batched_entropy(void)
+{
+	int cpu;
+	unsigned long flags;
+
+	write_lock_irqsave(&batched_entropy_reset_lock, flags);
+	for_each_possible_cpu (cpu) {
+		per_cpu_ptr(&batched_entropy_u32, cpu)->position = 0;
+		per_cpu_ptr(&batched_entropy_u64, cpu)->position = 0;
+	}
+	write_unlock_irqrestore(&batched_entropy_reset_lock, flags);
+}
+
 /**
  * randomize_page - Generate a random, page aligned address
  * @start:	The smallest acceptable address the caller will take.